It's polite to explain why you are unable to accept an invitation or offer.
For example:
I'd love to but I'm busy, I'm visiting my sister this weekend. 

Everyday English with Teacher Alyse

Not Accepting an Invitation

Everyone makes mistakes and it’s important to know what to say or write to structure an apology.

You should say what you’re apologizing for, admit the mistake you’ve made and explain what happened and the reason for your mistake. Say what you’ve done or what you’re going to do to correct the mistake.  

Apologizing and admitting fault

I’m so sorry I…
I have to apologize for…
I can imagine you must be very… about…
It was my fault/mistake.
I didn’t mean to…

Saying what you’re going to do about the mistake

In the future, I’ll ensure / make sure that I…
I’d like to make it up to you by…
Will you let me…to make up for my…mistake?

Dealing with your mistakes

Many of us have a preferred way of dealing with conflict. However, the way you respond to conflict should depend on the situation. So, take time to consider the situation and decide on the best way of dealing with that conflict. 

Giving in:
You’re probably right.
I’m happy to do it your way.

Avoiding:
I’d prefer not to talk about this now.
I’ll leave it to you to decide.

Forcing:
Let me tell you how this made me feel.
This is what I think we should do…

Compromising:
If I (do this) can you (do this)?
I’m happy to agree on (this) if you can (do that).

Working together:
I’d like to hear your side of things.
What do you think is the best way forward?

Follow me for more Everyday English. 
https://www.italki.com/en/teacher/13365914

Managing conflict phrases

Have to, don’t have to, and can’t
Use have to for general obligations and to show that something is necessary.
I have to work shifts.
You have to come on time.

Use don’t have to to show that something isn’t necessary.
You don’t have to wear a suit.

Use can’t to show that something is not allowed or isn’t a good idea.
You can’t lie in a job interview. 

Also use have to for strong recommendations. 
You have to do your research. 